v2.1.0

CHANGES:

This release has backwards incompatible changes.

• Infix functions are now available in Parser module itself.
• Add support for let operators - let+, let* ,and+ and and*.
• bind and map function are now labelled following base library
  dependency convention.
• Items in all_unit are now unit t rather than _ t following monad
  combinator convention in base library dependency.
• pure is now deprecated. Use return instead. This is to stay consistent
  with monad conventions in base library dependency.
• >|= is deprecated. Use >>| instead. This is to stay consistent with monad
  conventions in base library dependency.
• Removed map4 function.
• Add support for ppx_let.
• Deprecate Parser module. Use Reparse instead.